The present invention relates to methods for the manufacture of porous resin moldings, ultrafine fibers and nonwoven fabrics based on ultrafine fibers.
Heretofore, the following techniques are known for the manufacture of ultrafine-porous products from thermoplastic resins.
(1) The sintering process, in which a finely divided resin material is sintered;
(2) the solution process, which comprises dissolving a material resin in a solvent, adding a non-solvent thereto, coating a substrate, such as a nonwoven fabric, with the resulting mixture, and evaporating the solvent;
(3) the extraction process, which comprises mixing either a liquid or a solid into a material resin, molding the resin into a shaped article and extracting out the added liquid or solid;
(4) the drawing process, in which a crystalline polymer is formed into a film and stretched to give a porous product; and
(5) the process which comprises dispersing an inorganic salt in a material resin and dissolving out the inorganic salt with water.
Among these techniques, the extraction method includes the following versions. For example, Japanese Kokai Patent Publication No. 57-35906 discloses a method in which a substantial quantity of polyethylene glycol is added to a film-forming solution and, after the formation of film, the polyethylene glycol is extracted out to leave a porous product. Japanese Kokai Patent Publication No. 55-746 describes a process in which a polyolefin and a water-soluble substance such as pentaerythritol are mixed under heating and, after molding, the water-soluble substance is extracted with water. Furthermore, Japanese Kokai Patent Publication No. 61-212305 describes a process in which a polysulfone resin and a water-soluble carboxylic acid are mixed together and, after the formation of film, the carboxylic acid is extracted with water.
Aside from the above, Japanese Patent Publication No. 1-20262 discloses a process for production of porous polyester fiber in which a small proportion of a polyalkylene ether (the molecular weight of polyethylene oxide: 2,000,000) is blended with the polyester and, then, extracted out with alkali.
However, in practicing these prior art extractive processes for the manufacture of ultrafine-porous products, much labor and time are required in order that the extraction of polyethylene glycol, for instance, can be complete and thorough. Moreover, when a large amount of polyethylene glycol is used, excessive porosity is produced to seriously impair the mechanical strength of product film. Furthermore, when a polyethylene oxide of high molecular weight is employed, the dissolution rate of this substance with respect to water is so low that an aqueous solution of alkali must be employed for extraction, for instance. However, the extraction rate is not high enough and the disposal of spent alkaline liquor presents another problem.
Meanwhile, ultrafine fiber has been finding application in a diversity of uses, such as high-performance filters, artificial leather, nonwoven fabrics, noble woven or knitted garments, and so on.
For the manufacture of ultrafine fiber, there is known a method which comprises melt-spinning a polyester and either a polyamide or a polyolefin in a composite spinning fashion and, then, removing the polyamide or polyolefin with an acid or a solvent by way of selective dissolution. However, since the inter-polymer affinity is low, delamination tends to take place in the course of spinning, and in connection with the process for dissolution, corosion of the equipment and adverse effects on the physiology of man are constant causes for concern.
In the process employing a polyester copolymerized with a large proportion of polyethylene glycol as a segment to be subsequently removed by dissolution, a large amount of white powder is produced in the stage of false twisting to interfere with the operation.
Meanwhile, fabrics constructed with such ultrafine fiber are being used in an ever-increasing variety of uses, such as artificial leather, water-repellent and water vapor-permeable clothing, special filters, masks, high-heat insulations and so on.
The common technology for manufacturing such fabrics comprises extruding two different types of resin from a special spinneret nozzle to produce a composite fiber with a core-sheath arrangement or islet-ocean sectional pattern of polymers and, either after or before construction of a fabric, removing one of the component resins by extraction with an organic solvent or an aqueous acid or alkali solution. (cf. Japanese Kokai Patent Publication No. 54-27058)
However, these technologies have their own drawbacks, viz. (1) it takes a great deal of time and labor to thoroughly extract a resin, (2) the use of an organic solvent such as toluene, trichloroethylene or the like presents problems in terms of environment and safety and extraction with an acid or alkali involves a sophisticated procedure unless the resin is acid/alkali-degradable and a problem in connection with the disposal of spent liquor, (3) for the manufacture of nonwoven fabrics, the use of an adhesive resin for inter-fiber bonding is limited by the fact that the adhesive resin must not be soluble in the extraction solvent.